Engelsma MY †, Haenen OLM †
† National Reference Laboratory for Fish and Shellfish Diseases, Central
Institute for Animal Disease Control (CIDC-Lelystad), Wageningen University
and Research, Lelystad, The Netherlands
Collection and cultivation of shellfish as a food source has a long history. In the Netherlands the cultivation of mussels (Mytilus edulis), native oysters (Ostrea edulis) and Pacific oysters (Crassostrea gigas) and the fishing of cockles (Cerastoderma edule) from their natural populations are of particular commercial importance. Production of shellfish can be hampered by predation (crabs, starfish), by competition for food and space (slipper limpets) and by fouling (barnacles). With experience and good management these problems can be kept under control. More difficult to control are parasitic organisms as Mytilicola intestinalis, a copepod in the gut of mussels, which can reduce the growth rate. Others like Ostracoblabe implexa, a fungus, and Polydora sp., a polychaete worm, can penetrate the shell of oysters and can thereby reduce the market value of the shellfish.
The most severe disease problems for shellfish arise from a group of protists specialized in a parasitic lifestyle. In Western Europe, shellfish culture is especially threatened by Marteilia refringens, and culture of native oysters in particular by Bonamia ostreae. B. ostreae infects blood cells of the native oyster causing high mortality. In the Netherlands B. ostreae was introduced in 1980 in the Oosterschelde. In 1988 it was also detected in Lake Grevelingen. This caused a decline in the already reduced stocks of native oysters. At the moment the cultivation of Pacific oysters has largely replaced the culture of the native oyster. M. refringens has not been observed in the Netherlands since 1978.
Spread of shellfish diseases is mainly caused by transport and relaying of live shellfish. Once a disease is present in an area, possibilities for treatment are scarce. Hence, prevention of the spread of pathogens is the main focus of attention. Diagnosis of shellfish parasites is done primarily by histopathology, which facilitates detection of a wide range of parasites. In addition, electron microscopy and molecular techniques are used for a more sensitive and specific detection of pathogens. Recently, PCR and in situ hybridisation tests have been developed for detection of B. ostreae and are now used as confirmatory tests in diagnostics and for research applications. Such tests facilitate research into the life-cycles of these shellfish parasites, which are often not completely elucidated.
Edwardsiella tarda is a bacterium causing acute sepsis in mostly cultured eel, and Herpesvirus anguillae (HVA), originally from East-Asia, causes mortalities in eels, when they are under stress. Last year there were high eel mortalities in our country in the river Nieuwe Merwede during a very dry and hot summer month. The water had decreased to a minimum, and it had an extreme high Biological Oxygen Demand. A double infection of E. tarda and HVA was diagnosed in the stressed eels.
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